Valab 23 step attenuator musings

[grausch]

Thanks for the explanation. Still not sure exactly how the attentuator works, and I will need to think about this some more. Have found the Goldpoint website with a schematic which helped.

Resistance readings between input and ground show approximately 100k in all positions. This one does occasionally go higher or lower, highest I measured was 102 and lowest 95. I guess this is still within spec.

Ground and output will start off at 0 and then increase to 100k as I increase the steps and input and output will decrease as I increase the steps. However, the sum of these two readings will always be very close to the input and ground reading at each step. Measuring ground and output provided the easiest way of measuring the increases. Trying to measure input and output led to erratic readings, but this was not present on ground and output.

[Paul Birkeland]

Resistance readings between input and ground show approximately 100k in all positions. This one does occasionally go higher or lower, highest I measured was 102 and lowest 95. I guess this is still within spec.

Yeah, that's normal.  You can't always get the perfect pair of resistors for each step to give you both an even step size and exactly 100K. Maintaining step size is generally a lot more important than keeping the impedance perfectly constant.

[Doc B.]

As you turn the attenuator down the signal level gets shunted to ground thru the progressively lower resistance of the path to ground from the wiper. As you turn it up that resistance increases and thus the signal level the grid of the tube sees is bigger since the shunting effect to ground is reduced by the increasingly higher resistance from wiper to ground.

It's kinda like the attenuator is a T valve with an inlet for the signal, an outlet to the tube, and an outlet to the ground. And the signal is a liquid. When you turn the volume down, the signal spills out all over the ground rather than making it to the tube. So the level of signal that makes it to the tube through the other outlet of the valve is lower.

[mcandmar]

The blow diagram makes it easy to understand.  For example a five position 100k attenuator would look like,

Rin - Rg
100k - 0k
75k - 25k
50k - 50k
25k - 75k
0k - 100k

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2F3.bp.blogspot.com%2F_u_EoD7hGIPI%2FStdOM9kuEkI%2FAAAAAAAAACk%2F0-N5o8y90YE%2Fs400%2Fblock%2Bdiagram.JPG&hash=e0051499bc813073db01e38b0af9ab6905fe0f72)

[grausch]

Thank you all for the explanations,

I was pretty close to grasping this, but the T-Valve explanation makes this a lot easier to grasp. Rather than thinking of only one way for the electricity to flow, I need to realise that it can also flow to ground. As per high school science, electricity chooses the path of least resistance.

Am I right in assuming that since V=IR, as we increase the volume, the tube gets more current and therefore the volume increases? Thus, could the "level of signal" in Doc's post be replaced with current? Also, with the attentuator on anything other than a zero setting, does some of the electricity actually flow to ground, or is it current and resistance merely changed (again V=IR as per the above) and all of it flows to the tube?

Damn, I am now realising the extent of my ignorance when it comes to electricity...

[Doc B.]

signal level doesn't really correlate to current, it's about potential, i.e., voltage. That's what changes when the signal gets shunted. It's the level of the fluid that goes down. How much current flows when it goes down is pretty immaterial because it flows fast enough to change as fast as you turn the knob.

[Joshewah]

grausch, I've also received an attenuator with an upside down PCB.

If you wouldn't mind, would you be able to indicate on your first photo what ended up being L in, L out, R in and R out.

Mine is wired exactly like yours with the two white and black wires and the white and yellow striped wire.

I'm pretty sure I already have everything figured out with tracing the leads, but I'd just like some double verification. Thanks.

[grausch]

I would recommend doing PB's check and just marking them as he explained. Since the PCB is upside down, I was unsure if the rest of the attentuator was correct, so PB's method can helo you identify that.

On my attentuator, the top two were input, middle two ground and bottom two output. Left and right does not matter, just make sure the use the same side for each of the channels.